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<a name="Parameters-In-Guile"></a>
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Next: <a href="Progspaces-In-Guile.html#Progspaces-In-Guile" accesskey="n" rel="next">Progspaces In Guile</a>, Previous: <a href="Commands-In-Guile.html#Commands-In-Guile" accesskey="p" rel="prev">Commands In Guile</a>, Up: <a href="Guile-API.html#Guile-API" accesskey="u" rel="up">Guile API</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Parameters-In-Guile-1"></a>
<h4 class="subsubsection">23.3.3.12 Parameters In Guile</h4>

<a name="index-parameters-in-guile"></a>
<a name="index-guile-parameters"></a>
<a name="index-Parameter-1"></a>
<p>You can implement new <small>GDB</small> <em>parameters</em> using Guile
<a name="DOCF17" href="#FOOT17"><sup>17</sup></a>.
</p>
<p>There are many parameters that already exist and can be set in
<small>GDB</small>.  Two examples are: <code>set follow-fork</code> and
<code>set charset</code>.  Setting these parameters influences certain
behavior in <small>GDB</small>.  Similarly, you can define parameters that
can be used to influence behavior in custom Guile scripts and commands.
</p>
<p>A new parameter is defined with the <code>make-parameter</code> Guile function,
and added to <small>GDB</small> with the <code>register-parameter!</code> Guile function.
This two-step approach is taken to separate out the side-effect of adding
the parameter to <small>GDB</small> from <code>make-parameter</code>.
</p>
<p>Parameters are exposed to the user via the <code>set</code> and
<code>show</code> commands.  See <a href="Help.html#Help">Help</a>.
</p>
<dl>
<dt><a name="index-_0028make_002dparameter"></a>Scheme Procedure: <strong>(make-parameter</strong> <em>name <span class="roman">[</span>#:command-class command-class<span class="roman">]</span> <span class="roman">[</span>#:parameter-type parameter-type] <span class="roman">[</span>#:enum-list enum-list<span class="roman">]</span> <span class="roman">[</span>#:set-func set-func] <span class="roman">[</span>#:show-func show-func] <span class="roman">[</span>#:doc doc] <span class="roman">[</span>#:set-doc set-doc] <span class="roman">[</span>#:show-doc show-doc] <span class="roman">[</span>#:initial-value initial-value])</em></dt>
<dd>
<p>The argument <var>name</var> is the name of the new parameter.  If <var>name</var>
consists of multiple words, then the initial words are looked for as prefix
parameters.  An example of this can be illustrated with the
<code>set print</code> set of parameters.  If <var>name</var> is
<code>print foo</code>, then <code>print</code> will be searched as the prefix
parameter.  In this case the parameter can subsequently be accessed in
<small>GDB</small> as <code>set print foo</code>.
If <var>name</var> consists of multiple words, and no prefix parameter group
can be found, an exception is raised.
</p>
<p>The result is the <code>&lt;gdb:parameter&gt;</code> object representing the parameter.
The parameter is not usable until it has been registered with <small>GDB</small>
with <code>register-parameter!</code>.
</p>
<p>The rest of the arguments are optional.
</p>
<p>The argument <var>command-class</var> should be one of the &lsquo;<samp>COMMAND_</samp>&rsquo; constants
(see <a href="Commands-In-Guile.html#Commands-In-Guile">Commands In Guile</a>).  This argument tells <small>GDB</small> how to
categorize the new parameter in the help system.
The default is <code>COMMAND_NONE</code>.
</p>
<p>The argument <var>parameter-type</var> should be one of the &lsquo;<samp>PARAM_</samp>&rsquo; constants
defined below.  This argument tells <small>GDB</small> the type of the new
parameter; this information is used for input validation and
completion.  The default is <code>PARAM_BOOLEAN</code>.
</p>
<p>If <var>parameter-type</var> is <code>PARAM_ENUM</code>, then
<var>enum-list</var> must be a list of strings.  These strings
represent the possible values for the parameter.
</p>
<p>If <var>parameter-type</var> is not <code>PARAM_ENUM</code>, then the presence
of <var>enum-list</var> will cause an exception to be thrown.
</p>
<p>The argument <var>set-func</var> is a function of one argument: <var>self</var> which
is the <code>&lt;gdb:parameter&gt;</code> object representing the parameter.
<small>GDB</small> will call this function when a <var>parameter</var>&rsquo;s value has
been changed via the <code>set</code> API (for example, <kbd>set foo off</kbd>).
The value of the parameter has already been set to the new value.
This function must return a string to be displayed to the user.
<small>GDB</small> will add a trailing newline if the string is non-empty.
<small>GDB</small> generally doesn&rsquo;t print anything when a parameter is set,
thus typically this function should return &lsquo;<samp>&quot;&quot;</samp>&rsquo;.
A non-empty string result should typically be used for displaying warnings
and errors.
</p>
<p>The argument <var>show-func</var> is a function of two arguments: <var>self</var> which
is the <code>&lt;gdb:parameter&gt;</code> object representing the parameter, and
<var>svalue</var> which is the string representation of the current value.
<small>GDB</small> will call this function when a <var>parameter</var>&rsquo;s
<code>show</code> API has been invoked (for example, <kbd>show foo</kbd>).
This function must return a string, and will be displayed to the user.
<small>GDB</small> will add a trailing newline.
</p>
<p>The argument <var>doc</var> is the help text for the new parameter.
If there is no documentation string, a default value is used.
</p>
<p>The argument <var>set-doc</var> is the help text for this parameter&rsquo;s
<code>set</code> command.
</p>
<p>The argument <var>show-doc</var> is the help text for this parameter&rsquo;s
<code>show</code> command.
</p>
<p>The argument <var>initial-value</var> specifies the initial value of the parameter.
If it is a function, it takes one parameter, the <code>&lt;gdb:parameter&gt;</code>
object and its result is used as the initial value of the parameter.
The initial value must be valid for the parameter type,
otherwise an exception is thrown.
</p></dd></dl>

<dl>
<dt><a name="index-register_002dparameter_0021"></a>Scheme Procedure: <strong>register-parameter!</strong> <em>parameter</em></dt>
<dd><p>Add <var>parameter</var>, a <code>&lt;gdb:parameter&gt;</code> object, to <small>GDB</small>&rsquo;s
list of parameters.
It is an error to register a parameter more than once.
The result is unspecified.
</p></dd></dl>

<dl>
<dt><a name="index-parameter_003f"></a>Scheme Procedure: <strong>parameter?</strong> <em>object</em></dt>
<dd><p>Return <code>#t</code> if <var>object</var> is a <code>&lt;gdb:parameter&gt;</code> object.
Otherwise return <code>#f</code>.
</p></dd></dl>

<dl>
<dt><a name="index-parameter_002dvalue"></a>Scheme Procedure: <strong>parameter-value</strong> <em>parameter</em></dt>
<dd><p>Return the value of <var>parameter</var> which may either be
a <code>&lt;gdb:parameter&gt;</code> object or a string naming the parameter.
</p></dd></dl>

<dl>
<dt><a name="index-set_002dparameter_002dvalue_0021"></a>Scheme Procedure: <strong>set-parameter-value!</strong> <em>parameter new-value</em></dt>
<dd><p>Assign <var>parameter</var> the value of <var>new-value</var>.
The argument <var>parameter</var> must be an object of type <code>&lt;gdb:parameter&gt;</code>.
<small>GDB</small> does validation when assignments are made.
</p></dd></dl>

<p>When a new parameter is defined, its type must be specified.  The
available types are represented by constants defined in the <code>gdb</code>
module:
</p>
<dl compact="compact">
<dt><code>PARAM_BOOLEAN</code>
<a name="index-PARAM_005fBOOLEAN-1"></a>
</dt>
<dd><p>The value is a plain boolean.  The Guile boolean values, <code>#t</code>
and <code>#f</code> are the only valid values.
</p>
</dd>
<dt><code>PARAM_AUTO_BOOLEAN</code>
<a name="index-PARAM_005fAUTO_005fBOOLEAN-1"></a>
</dt>
<dd><p>The value has three possible states: true, false, and &lsquo;<samp>auto</samp>&rsquo;.  In
Guile, true and false are represented using boolean constants, and
&lsquo;<samp>auto</samp>&rsquo; is represented using <code>#:auto</code>.
</p>
</dd>
<dt><code>PARAM_UINTEGER</code>
<a name="index-PARAM_005fUINTEGER-1"></a>
</dt>
<dd><p>The value is an unsigned integer.  The value of 0 should be
interpreted to mean &ldquo;unlimited&rdquo;.
</p>
</dd>
<dt><code>PARAM_ZINTEGER</code>
<a name="index-PARAM_005fZINTEGER-1"></a>
</dt>
<dd><p>The value is an integer.
</p>
</dd>
<dt><code>PARAM_ZUINTEGER</code>
<a name="index-PARAM_005fZUINTEGER"></a>
</dt>
<dd><p>The value is an unsigned integer.
</p>
</dd>
<dt><code>PARAM_ZUINTEGER_UNLIMITED</code>
<a name="index-PARAM_005fZUINTEGER_005fUNLIMITED"></a>
</dt>
<dd><p>The value is an integer in the range &lsquo;<samp>[0, INT_MAX]</samp>&rsquo;.
A value of &lsquo;<samp>-1</samp>&rsquo; means &ldquo;unlimited&rdquo;, and other negative
numbers are not allowed.
</p>
</dd>
<dt><code>PARAM_STRING</code>
<a name="index-PARAM_005fSTRING-1"></a>
</dt>
<dd><p>The value is a string.  When the user modifies the string, any escape
sequences, such as &lsquo;<samp>\t</samp>&rsquo;, &lsquo;<samp>\f</samp>&rsquo;, and octal escapes, are
translated into corresponding characters and encoded into the current
host charset.
</p>
</dd>
<dt><code>PARAM_STRING_NOESCAPE</code>
<a name="index-PARAM_005fSTRING_005fNOESCAPE-1"></a>
</dt>
<dd><p>The value is a string.  When the user modifies the string, escapes are
passed through untranslated.
</p>
</dd>
<dt><code>PARAM_OPTIONAL_FILENAME</code>
<a name="index-PARAM_005fOPTIONAL_005fFILENAME-1"></a>
</dt>
<dd><p>The value is a either a filename (a string), or <code>#f</code>.
</p>
</dd>
<dt><code>PARAM_FILENAME</code>
<a name="index-PARAM_005fFILENAME-1"></a>
</dt>
<dd><p>The value is a filename.  This is just like
<code>PARAM_STRING_NOESCAPE</code>, but uses file names for completion.
</p>
</dd>
<dt><code>PARAM_ENUM</code>
<a name="index-PARAM_005fENUM-1"></a>
</dt>
<dd><p>The value is a string, which must be one of a collection of string
constants provided when the parameter is created.
</p></dd>
</dl>

<div class="footnote">
<hr>
<h4 class="footnotes-heading">Footnotes</h4>

<h3><a name="FOOT17" href="#DOCF17">(17)</a></h3>
<p>Note that <small>GDB</small> parameters must not be confused with
Guileâs parameter objects (see <a href="http://www.gnu.org/software/guile/manual/html_node/Parameters.html#Parameters">Parameters</a> in <cite>GNU Guile
Reference Manual</cite>).</p>
</div>
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